The invention relates to a piezoelectric actuator for high pressure media which flow around it.
In order to reduce the emissions of internal combustion engines, injection systems are increasingly used for the fuel supply, in which fuel is conveyed with the aid of a high-pressure pump into an accumulator and is injected from there into the combustion space with the aid of an injector. The activation of the injector takes place by means of an electrically activated actuator, preferably a piezoelectric actuator. On account of the very short switching times of such piezoelectric actuators, the injection operations can be controlled and metered with high accuracy; in particular, when piezoelectric actuators are employed, a plurality of nozzle needle strokes (injection operations) per engine revolution are possible.
A piezoelectric actuator contains a piezo element consisting of quartz ceramic or PZT ceramic (lead/zirconate/titanate ceramic), the active main faces of which are connected to an actuator cover and the actuator bottom. By an electrical voltage being applied to the actuator cover or actuator bottom, the length of the piezo element can be varied. This length change is transmitted, during operation, to a valve in the fuel injector.
The problem is that the piezoelectric actuator is often in direct contact with the media or fuel, since it is arranged in the pressure space of the injector and fuel under a high hydrostatic pressure washes around it there. In order to prevent oil or fuel from penetrating into the interior of the piezoelectric actuator, therefore, the latter has to be provided with a protective or sealing-off arrangement. This may, for example, be a sleeve-shaped housing jacket which is fastened on the end face to the actuator bottom and to the actuator cover respectively. In the German patent document DE 102 30 032 A1, to protect the piezoelectric actuator against media flowing around it, it is proposed to provide the actuator with a sheathing composed of an electrically insulating, flaccid and/or elastic material which surrounds the piezo element. Furthermore, it is known from (German patent application 10 2006 012 845.1) to configure the sheathing as a shrunk-on hose which is firmly pressed on the end faces against the outer circumference of the actuator bottom and of the actuator cover with the aid of peripheral ring elements.
By means of a sheathing composed of an electrically insulating material, in particular of a shrunk-on film, the piezo element can, indeed, be protected against the fuel which washes around the actuator (and which is under high pressure). However, a completely diffusion-tight sheathing often cannot be implemented by means of such polymer films.
One object of the invention, therefore, is to provide a piezoelectric actuator of the type described above, in which the piezo element of the actuator is effectively protected against flowing-around media.
This and other objects and advantages are achieved by the piezoelectric actuator according to the invention, in which the sheathing consists, at least in portions, of a composite material with at least two layers, at least one layer consisting of a metallic material and at least one layer consisting of a polymer. Such a composite material is electrically conductive; the sheathing can therefore be wired up electrically (in particular, by grounding the sheathing) in such a way that it is impossible for ions to penetrate the sheathing. The sheathing thus constitutes a diffusion-tight protective layer and protects the piezo element against penetrating liquids, in particular against fuel, water and ions.
On the other hand, the polymer layer of the composite material can be configured in such a way that it ensures a chemical passivity of the sheathing.
The sheathing has to satisfy high requirements: it must protect the piezo element effectively against penetrating media (fuel, water, ions, etc.) and, particularly also in the joining region between the sheathing film and end caps, ensure permanent diffusion tightness. Furthermore, even under cyclic load, it must be insensitive to the hydrostatic pressure acting in the pressure space, that is to say must possess a sufficient expansion capacity and high elasticity. Such properties must be ensured within the overall temperature range of between −40° C. and 120° C.
In order to fulfill these high requirements, the use of a composite material for the sheathing offers the possibility of suitably selecting or adapting the number, sequence and thickness of the individual layers. Furthermore, the metal layer and/or the polymer layer of the sheathing may be provided with reinforcing fabrics and/or fibers. Moreover, the composite material of the sheathing may have, in addition to the at least one polymer layer and one metal layer, further layers (consisting, in particular, of paper and/or cardboard and/or fabric and/or fibers) which in this case each perform a particular function. Layers or layer systems into which ion getters are implanted may also be used. In an advantageous combination of individual layers, it is possible to cover the entire requirement spectrum of the sheathing.
The metallic layer or metallic layers of the composite material may, in particular, be roll-bonded or electrodeposited or generated by means of physical vapor deposition (PVD) or chemical vapor deposition (CVD).
A major advantage of using composite materials is the possibility of applying or processing coatings which could not be applied or could not be used as individual layers standing alone so as to ensure process reliability. One example of this is metal films consisting of pure aluminum or aluminum alloys, which can be produced in a film thickness of 6 μm, but, with this low layer thickness, cannot fulfill the requirements placed on an actuator sheathing or cannot be applied reliably; if, however, such a thin aluminum film is drawn onto a tear-resistant, expandable polymer film, the composite material which in this case occurs is suitable for actuator sheathing.
Furthermore, by an advantageous combination and sequence of the various layers in the composite material, it is possible to achieve properties or functions which are possible only due to this combination. Thus, for example, an aluminum-coated polymer film has an increased expansion and shear capacity, as compared with a pure aluminum film.
In an advantageous refinement of the invention, the sheathing comprises a (sheathing) film composed of a composite material, which film is connected, diffusion-tight, to the end caps of the actuator. In this context, “film” is to be understood as meaning a sheet-like, flexible material portion expandable or elastic within certain limits and having a small wall thickness. The sheathing film seals off the inner space of the sleeve with respect to the outside space and therefore protects the piezo element from the flowing-around media, such as fuel or fuel constituents and water. On account of its elastic properties, it allows pressure compensation between the actuator inner space, enclosed by the sheathing film, and the surroundings.
In a further advantageous refinement of the invention, the sheathing comprises a sleeve having locally delimited perforations which are spanned by an elastic and expandable film connected sealingly to the sleeve. The film seals off the inner space of the sleeve with respect to the outside space and therefore protects the piezo element against the flowing-around media, such as fuel or fuel constituents and water. At the same time, because of its elastic expandable properties, the film allows pressure compensation between the two spaces even in the case of high ambient pressures (up to 2000 bar). The sleeve and/or the film in this case consist/consists of a composite material.
As is known from German patent document 10 2006 012 845.1, the sheathing may be fastened to the end caps of the actuator by means of ring-shaped clamping elements. Alternatively, the sheathing may be adhesively bonded to the end caps or be connected to the end caps by means of a soldered or welded joint (for example, WIG welding, laser or electron beam welding).
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.